The principles of total reflection and refraction have been used rather frequently individually and jointly in various optical instruments. Several of these devices were developed and applied for the sensing of levels of liquids and to indicate the absence and the presence, respectively, of a liquid in a hydraulic system with respect to a predetermined level. The indication was obtained through visual observation, auxiliary electric transceiver circuits, or any combinations thereof. The indications were provided by means of light vs. dark signal areas, displays of contrasting colors, and other comparable, traditional arrangements. As the complexity of the to be monitored systems increased, those fairly primitive, traditional indicators no longer present a specific information. For example, a green light, usually standing for a clear condition, did not signify as to whether the sensor's condition or that of the liquid system was all right. Even more uncertain was a sensing device which displayed a signal in case of a danger, but no signal when conditions were safe; obviously, no signal could have also been the result of a defective signal circuit thereby precluding corrective action for the monitored liquid system.
Monitoring equipment of the aforementioned general characteristics has been shown and described in various publications which include, but may not be limited to the following references:
Nicholas P. Chironis: MACHINE DEVICES AND INSTRUMENTATION; McGraw-Hill Book Company, New York, 1966, Library of Congress Catalog Card Number 66-18474; Aspect Sensor, p. 283; Oil level detector, p. 335. PA0 Periodical "CONTROL ENGINEERING", August 1974: Product data on "LIQUID SENSOR" of Genelco, Inc., Dallas, TX.
The devices covered by the listed references contain the undesirable features which this invention intends to overcome in accordance with the following equipment description.